delete unused GYRO_SAMPLERATE_DEFAULT

src/drivers/drv_gyro.h

src/drivers/imu/fxas21002c/fxas21002c.cpp

src/drivers/imu/l3gd20/l3gd20.cpp

src/drivers/imu/mpu6000/mpu6000.cpp

src/drivers/imu/mpu9250/mpu9250.cpp

src/modules/simulator/gyrosim/gyrosim.cpp